WASHINGTON — Want to know all the airline fees you could face as you compare fares on the Web or at a travel agency? The Department of Transportation might make that happen.

The Transportation Department is considering whether to force airlines to disclose all ancillary fees through every potential sales channel — and not just through airline websites. Consumer groups and travel agencies say the rule is needed to allow travelers to truly compare fares.

Even as airlines began several years ago to “unbundle” services such as checked baggage, most carriers don’t disclose other potential fees through third-party travel websites and travel agencies. The lack of transparency can make fares look artificially low, although carriers say unbundling saves money for some customers.

“The consumer here is being harmed, and the government has a role to step in,” Bill McGee, a contributing editor to Consumer Reports, said at an event Thursday in Washington. “There is a growing mistrust among consumers for the whole process. That is going to hurt the airline industry and the whole economy.”

Airlines say the regulation would advance the notion of plain-vanilla airline fares, thwarting efforts to distinguish airline offers from others. The rule also could give a leg-up to global technology firms that market fares for multiple carriers through websites such as Travelocity and Orbitz, airlines say.

Ancillary fees have become one way for carriers to improve their lackluster financial performance. Carriers reaped about $1.75 billion from bag fees during the first six months of this year, according to the Transportation Department.

The Transportation Department finalized a regulation earlier this year that required airlines and ticket agents to disclose bag fees when customers are quoted airfare. Typically the customer is shown a long list of bag-fee policies for each airline.

Now consumer groups and travel agents want regulators to require more complete transparency of fees for other unbundled services, such as overweight luggage, seat assignments and priority boarding. To do so, regulators would have to conclude that the current practice of disclosure is unfair and deceptive.

The department is scheduled to announce a new proposal in January, which would kick off a 30-day public comment period.

Airlines say they already disclose all potential fees on their own websites. They want to be left alone to negotiate deals over sending that data to the technology companies — including Southlake-based Sabre Holdings and Atlanta-based Travelport — that power online travel websites and travel-agency systems.

‘No control’

Ultimately, carriers also want to personalize ticket marketing in a way that third-party vendors don’t offer, said Sharon Pinkerton, senior vice president for legislative and regulatory policy at Airlines for America, the airline trade group.

“The problem is forcing us to give it to them so they can display it in a manner that we have absolutely no control over,” Pinkerton said.

Airlines pay about $7 billion a year to the technology companies, known as global distribution systems (GDS), which are responsible for 50 percent of worldwide ticket sales. The carriers don’t want to pay more to provide the fees.

“If you go in and you force carriers to give GDSs the information, we have lost all of our ability to negotiate cost,” Pinkerton said.

The tech companies say cost isn’t the issue — the portion of an airfare that stems from their services is just 2 percent, according to Simon Gros, chairman of the Travel Technology Association and a vice president for government affairs at Travelport.

Gros says the technology firms have inked agreements with many foreign airlines to disclose ancillary fees. Sabre says it’s told U.S. airlines that it won’t charge them extra to display information about fees for baggage, premium seats and priority boarding.

“The ability to do that seamless comparison shopping is not there today, when information is missing from certain sales channels,” said Chris Kroeger, senior vice president for marketing at Sabre Travel Network.

‘Market-share play’

Some travel officials say airlines withhold the information because they want to book more tickets through their own systems, which would reduce their cost of ticket sales. (Dallas-based Southwest Airlines avoids that cost by selling tickets only through its own agents and website.)

“I personally believe the core reason is they want, for as long as they can get away with it, to drive customers to their own websites,” said Paul M. Ruden, senior vice president for legal and industry affairs at the American Society of Travel Agents. “It’s a market-share play.”

Airlines also worry about disclosing the fees through third-party websites and travel agencies if their competitors don’t do the same.

“If I went over the top and disclosed, and my competitors didn’t, would I look more expensive?” said Barry Biffle, executive vice president and chief marketing officer of Spirit Airlines, a low-fare carrier that charges for both checked and carry-on bags.

The Transportation Department will have the final say about that. Transportation Secretary Ray LaHood has already approved two pro-consumer rules, which put a three-hour limit on tarmac delays and require bag fees to be disclosed with fare quotes, among other things.

Some insiders say regulation extending that transparency to seats and boarding is the next step, because consumers expect those things to be part of the ticket. The rule also could require that consumers be able to pay for those fees when they buy a ticket, instead of at the airport.

“The pushback from consumers on fees and fee transparency is far and away the No. 1 complaint among passengers,” McGee said. “It’s clear that if the industry won’t deal with this, the DOT will.”